Archived posting to the Leica Users Group, 2007/03/14

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Subject: [Leica] DNG: Not Just 8-bit
From: robert.rose at (Robert Rose)
Date: Wed Mar 14 20:57:23 2007

 A detailed article in LFI 2/2007 explains that the DNG file is
not a true 8-bit file.  Rather it is (my words) a logarithmically quantized
14-bit file, allowing significant 6:1 file compression (a 10 Mb DNG file
opens to 59 Mb)

It works like this (my Cliff Notes version):

A true 8-bit file will quantize the luminance values into 2^8 = 256 equally
spaced values.  The key takeaway is that the steps are equally spaced,
shadow to highlight.

The DNG file starts with 2^14 = 16,384 steps, and then chooses 256 values,
logarithmically spaced so that the shadows get most of the steps, and the
highlights less.  This lets the DNG use only 8-bits to encode the file. The
steps can be interpolated back to 14 bits with the Adobe DNG converter.  "It
interprets the 8-bit data as an index to a table containing the actual tonal
values in 14-bit resolution."

Confused?  What it all boils down to is that the M8's DNG files should get
better tonal separation in the shadow areas than other camera RAW formats
that use 12-bits, with acceptable separation on the highlights.

Bob Rose

Replies: Reply from hoppyman at (G Hopkinson) ([Leica] DNG: Not Just 8-bit)